Optical readers, such as bar code readers, are now quite common. Typically, a bar code comprises a series of encoded symbols, and each symbol consists of a series of light and dark regions, typically in the form of rectangles. The widths of the dark regions, the hats, and/or the widths of the light spaces between the bars indicate the encoded information. A bar code reader illuminates the code and senses light reflected from the code to detect the widths and spacings of the code symbols and derive the encoded data.
Bar code reading type data input systems improve the efficiency and accuracy of data input for a wide variety of applications. The ease of data input in such systems facilitates more frequent and detailed data input, for example to provide efficient inventories, tracking of work in progress, etc. To achieve these advantages, however, users or employees must be willing to consistently use the bar code readers. The readers therefore must be easy and convenient to operate.
A variety of scanning devices are known. One particularly advantageous type of reader is an optical scanner which scans a beam of light, such as a laser beam, across the symbols.
Laser scanner systems and components of the type exemplified by U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,593,186; 4,496,831; 4,409,470; 4,460,120; 4,607,156; 4,673,803; 4,736,095; 4,758,717; 4,816,660; 4,808,804; 4,816,661; 4,760,248; 4,871,904; 4,806,742; and 4,845,350, as well as U.S. application Ser. Nos. 07/148,669 and 07/147,708--all of these patents and patent applications being owned by the assignee of the instant invention and being incorporated by reference herein--have generally been designed to read indicia having parts of different light reflectivity, e.g., bar code symbols, particularly of the Universal Product Code (UPC) type, at a certain working or reading distance from a hand-held or stationary scanner.
In a typical optical scanner system, a light source such as a laser generates a light beam which is optically modified to form a beam spot of a certain size at the working distance and is directed by optical components along a light path toward a bar code symbol located in the vicinity of the working distance for reflection from the symbol. A photodetector having a field of view extending across and slightly past the symbol detects light of variable intensity reflected off the symbol and generates electrical signals indicative of the detected light. A scanning component is situated in the light path. The scanning component may either sweep the beam spot across the symbol and trace a scan line across and past the symbol, or scan the field of view of the photodetector, or do both.
A digitizer, associated with or included in the scanner, processes the analog signal to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and the threshold value set by the digitizer determines what points of the analog signal represent bar edges. The pulse signal from the digitizer is applied to a decoder. The decoder first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyzes the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard the scanned symbol conforms to. This recognition of the standard is typically referred to as autodiscrimination.
Often the particular application for the bar code reader requires that the optical scanner be constructed in the form of a handheld unit, typically in the form of a pistol. The user points the scanner at the object and operates a trigger to activate the scanning of the bar code. In these handheld units, a light source, such as a visible laser diode (hereinafter VLD) emits a beam of light. To scan the beam across an image, the beam is reflected from a mirror and the mirror is moved in an oscillatory fashion. The oscillation of the mirror causes the reflected beam to scan back and forth in a desired pattern. For example, U.S. Pat. No. 4,251,798 discloses a rotating polygon having a planar mirror at each side, each mirror tracing a scan line across the symbol. U.S. Pat. Nos. 4,387,297 and 4,409,470 both employ a planar mirror which is repetitively and reciprocally driven in alternate circumferential directions about a drive shaft on which the mirror is mounted. U.S. Pat. No. 4,816,660 discloses a multi-mirror construction composed of a generally concave mirror portion and a generally planar mirror portion. The multi-mirror construction is repetitively reciprocally driven in alternate circumferential directions about a drive shaft on which the multi-mirror construction is mounted.
In optical scanners of the type discussed above, the mirror and means to oscillate the mirror add size and weight to the handheld scanner. In applications involving protracted use, a large heavy handheld unit can produce fatigue when use of the scanner produces fatigue or is in some way inconvenient, the user is reluctant to operate the scanner, which defeats the data gathering purposes for which bar code systems are intended.
Another problem with scanners of the type discussed above relates to the scan field and the size of the spot at the point where the beam impacts on the information. In scanning applications, such as reading bar codes, it is important to maintain the spot size of the beam on the scanned surface essentially at a constant across the entire length of the scanned information. Lasers emit a narrow core of radiation referred to as the laser beam. The beam cross-section varies as the laser beam propagates, particularly after focusing thereof by a lens. The minimum beam diameter is called the "beam waist." In the prior art, the scanning element, e.g. the moveable mirror, typically is positioned after the objective lens so as to scan the focused beam. Such scanning is referred to as "post objective" scanning. The objective lens in such scanners only needs to be slightly larger than the beam, and the scanning mirror can be somewhat smaller than the beam diameter. The mirror rotation, however, produces a curved or arcuate scan of the focal point of the beam. This curved scan field means that the focal point of the beam actually moves into and out of the plane of the information being scanned, and the part of the beam which impacts the plane of the information changes. Consequently, the spot size changes substantially from point to point along the scan line on the information surface. Such variations in spot size cause the intensity of light reflection sensed by the photodetector to vary, which may interfere with accurate sensing of reflection variations due to variations in the scanned information.
In non-laser scanning systems of the type exemplified by U.S. Pat. No. 4,578,571, a non-laser light emitting diode, an optical assembly, a photodetector, and electronic preamplifier/filter circuitry are all fixedly mounted on a common support that is connected to a cantilevered bimorph which is reciprocally driven to jointly move all the aforementioned components back and forth over a bar code symbol to be scanned. The large volume and heavy mass of all the commonly mounted non-laser system components require the expenditure of a great deal of power for the driver. This is not altogether practical in those cases, e.g. battery-powered operation, where power consumption is to be held to a minimum. Also, moving only one or more non-mirrored system components relative to another for conserving power was heretofore not considered desirable, because of optical alignment problems.
The components for the light scanning system, including the light source, optics, photo-detector, scanning component and an electrical conductor, can be mounted together in a common assembly to constitute a compact, lightweight, scan module. Such a scan module is mounted in an interchangeable, modular manner in housings of different configurations. For example, the housing can be hand-held and shaped as a cylinder in a so-called flashlight-type configuration, or shaped as a box, or shaped with a gun-like configuration. The housing can be mounted on the back of an operator's arm (see, for example, U.S. Pat. No. 4,766,299) or on one or more fingers of the operator's hand, typically with the aid of a strap, a clip, or a glove. The housing can be mounted on a countertop workstation. The housing can be mounted in a mobile cart, or shopping cart, or, in some cases, even in a stationary installation.
Commonly assigned application Ser. No. 07/193,265 filed May 11, 1988 discloses a mirrorless optical scanner, and application Ser. No. 07/699,417 filed on May 13, 1991 discloses incorporation of such a scanner in a modular scanner component system facilitating use of the scanner in a variety of different housing configurations.
A need still exists, however, to further reduce the size and weight of the scanner unit and to provide a particularly convenient scanner system. The mass of the moving components should be as low as possible to minimize the power required to produce the scanning movement and to facilitate operation at high scanning speeds.